Leading article: The bells of New Orleans toll for the whole planet

Three years ago President Bush received a letter from the then Mayor of New Orleans pleading with him to reverse his position on global warming. Along with the heads of 74 other coastal cities around the world - including 15 in the United States - he wrote to "underline the dangers our communities are facing from global climate change" and urged him to support the Kyoto Protocol. The present mayor added just this summer that global warming "threatened the very existence of New Orleans". No doubt these pleas went into a White House waste-paper basket, there to join a mass of other prophetic warnings of the disaster that has now overtaken the Big Easy. For, despite the President's insistence that no one "anticipated the breach of the levees", this has been the most predicted catastrophe in history.

In the event, even a dramatic change of heart on Kyoto by President Bush would have been too late to save New Orleans - though heeding calls to raise the levees, rather than slashing spending on them, might well have done. The world's natural systems have massive time-lags built into them - so that the warming we are seeing now reflects the pollution of decades ago. But, by the same token, his success in holding up global action on climate change - and his persistent refusal to tackle the United States' overwhelming contribution to it - is storing up even greater disasters for the future.

No one can honestly say whether or not Katrina was caused by global warming, but there is widespread agreement that continued warming will bring even more vicious hurricanes in future. Worse, top scientists meeting in Exeter earlier this year concluded that the world has probably only a decade to bring the emissions under control if we are to avoid such unthinkable disasters as the failure of the Gulf Stream. As the United States blocks progress, these crucial years tick by.

Much of the problem lies in those time-lags. If pollution caused immediate disasters we would move heaven and earth to cut it. Because it does not, reasons can always be found for delay, often encouraged by the most polluting industries. It is easy for them to produce - usually highly inflated - reckonings of what it would cost, in cash and jobs, to change. These are easily countered - study after study shows that addressing global warming would produce more prosperity not less - but even false warnings of imminent pain often carry more weight.

New Orleans can change that. There can no longer be any cause for doubt of the horrendous cost - in both lives and hard cash - that neglect will cause. Even before last week there were many signs that the American public, industry, and even much of the Republican Party was parting company with the President on this issue; the deep - and well deserved - political damage that the disaster has done him will surely accelerate the process. But not just America needs to head the warning. The bells of New Orleans toll for all of us - and Katrina must prove a turning point for us all.

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